After researching various types of castles, and mainly looking at gothic architecture, here are a couple of sketches drawn in an attempt to come up with the shape of Donkey Skin’s castle. While the first sketches of gothic architecture are more elaborate; the very rough ones are mainly research on shapes and particular features.
A finished drawing of the possible look of the castle:
Looking at fairytale films currently made, I noticed that castles are mainly fantasy/CGI rather than location. The idea is to keep elements of real castles, thus gothic architectural features; yet the design needs to be new and original, hinting to the audience that our story does not take place anywhere they know.